Field Emission Electron Source, Method of Producing Same, and Electron Beam Device Using Same

The present invention relates to a field emission electron source of an electron beam device such as an electron microscope, a method for producing the field emission electron source, and an electron beam device using the field emission electron source.

An electron microscope has spatial resolution exceeding an optical limit and can perform observation and a composition analysis for a microstructure in the order of nm to pm. Therefore, the electron microscope is widely used in engineering fields such as material, physics, medicine, biology, electric, and machine. The electron microscope includes a scanning electron microscope (SEM) serving as a device capable of easily observing a sample surface.

An electron source used in an electron beam device such as a scanning electron microscope includes a thermal electron source (a thermionic emitter: TE), a field emission electron source (a field emitter: FE), and a Schottky emission electron source (a Schottky emitter: SE). Among these electron sources, the field emission electron source (FE) can emit an electron beam having good monochromaticity and high luminance, can reduce chromatic aberration in an electron optical system, and is used as an electron source for a scanning electron microscope of high spatial resolution. A W tip using a {310} crystal plane of tungsten in which a distal end of a needle-shaped electrode (a tip) is sharpened is widely used as the field emission electron source.

shows an energy diagram of an emission principle of the field emission electron source. By concentrating an external electric field F at the distal end of the W tip, a high electric field is applied, and electrons e at the W tip are quantum mechanically transmitted through an energy barrier that is effectively thinned and are emitted into vacuum. Since such an operation can be performed at room temperature, an energy full width at half maximum ΔEof the drawn electrons e is as narrow as about 0.3 eV. Further, since an electron beam having a high density is emitted from a narrow electron emission surface at a very sharp distal end of the tip, such a field emission electron source has a feature of high luminance of 10(A/cmsr).

In the field emission electron source, since the energy full width at half maximum ΔE is narrowed and luminance B is increased, a field emission electron source using a nanowire of hexaboride, such as LaB, which has a work function Φ lower than W, is proposed (for example, PTL 1). Since a work function barrier of the hexaboride is lower than that of W, electrons can be transmitted by a lower electric field and can be field-emitted, and the energy full width at half maximum ΔEcan be further reduced.

Previously, the inventors developed and disclosed a cold field emission electron source (a cold field emitter, CFE) in which a hexaboride single crystal such as CeBproduced by a floating zone method or the like was used, a distal end of the hexaboride single crystal was shaped into a hemispherical shape by electrolytic polishing, field evaporation, or the like, and further heating processing was performed at 700° C. to 1400° C. to form a {310} crystal plane of CeBhaving a low work function, and field emission was performed at room temperature (PTL 2). The field emission electron source of the hexaboride single crystal has better monochromaticity than a field emission electron source using W in the related art, and when a ratio J/It of a radiation angle current density J(μA/sr) to a total current It (μA) is 6 (1/sr) to(1/sr) or more, the radiation angle current density J(μA/sr) can be increased. According to this invention, in particular, it is possible to improve chromatic aberration of a scanning electron microscope at a low acceleration voltage, and it is possible to observe a polar surface of a sample and achieve high spatial resolution observation for a light element substance such as a carbon-based compound. In addition, since a size of the hexaboride single crystal produced by the floating zone method or the like is about 0.1 mm to several mm, the hexaboride single crystal can be assembled to an electron source using a human hand or a machine, and the hexaboride single crystal has an advantage of being more inexpensive, more convenient, and having higher yield than an electron source that uses a nanowire having a diameter of several tens to several hundreds of nm.

Further, as a result of studies of the inventors, it was found that when a (100) plane facet was formed on a top portion of a tip of a hexaboride single crystal having a <100> axis of a hexaboride single crystal, and field emission electrons from the (100) plane served as a probe, stability of an emission current was higher than that of field emission electrons from a {310} plane. The reason is that a work function of the (100) plane is slightly higher than that of the {310} plane, the work function is less likely to be affected by an adsorption gas, an atomic area density is higher than that of the {310} plane which makes a structure stable and prevents atom vibration, heating makes it easier to form a flat facet with a large area and reduce an electric field concentration level, further local changes in the work function caused by gas adsorption and desorption during field electron emission are averaged over a large surface of the facet and an overall fluctuation is reduced, and the like. Therefore, when the (100) plane facet is formed at the distal end of the tip of the hexaboride single crystal with the <100> axis, and an electron beam emitted from the (100) plane serves as a probe, current stability can be improved.

However, there are problems that since the work function is high and a concentration level of an electric field is lowered, the (100) plane is less likely to emit electrons as compared with the {310} plane, and since electrons emitted from planes such as the {310} plane formed on side portions of the tip around the (100) plane are wasted outside an optical axis of an electron microscope, a ratio of the radiation angle current density J(μA/sr) to the total current It (μA) is significantly reduced to less than 1 (1/sr).

When the total current It (μA) is too large, wasted electrons emitted to the outside of the optical axis are radiated onto an extraction electrode or the like in the electron microscope to generate an electron beam stimulation desorption gas from a surface of the electrode. The electron beam stimulation desorption gas is incident and adsorbed on a surface of a field emission electron source, which causes a change in the work function and impairs current stability.

Therefore, as a result of intensive studies of inventors to solve this problem, it was disclosed that a stable electron source can be achieved by forming a (100) plane top facet at a distal end of a tip of a hexaboride single crystal with a <100> axis for forming the electron source, the (100) plane top facet being surrounded by side facets including at least four {n11} planes having a high work function and at least four {n10} planes having a low work function, n being an integer of 1, 2, or 3, and by setting a total area of {n11} plane side facets to be larger than a total area of {n10} plane side facets, an amount of unnecessary current emitted from the side facets can be reduced, and a ratio of a radiation angle current density J(μA/sr) of a probe current extracted from an electron emission portion of a top (100) plane of the electron source at the tip of the hexaboride single crystal to a total current It (μA) emitted from the electron source can be increased to 2.6 (1/sr) to 4 (1/sr), and a current fluctuation is smaller than that in the related art (PTL 3).

As described above in the background art, in the field emission electron source using the hexaboride single crystal such as CeB, when the field emission electrons from the (100) plane serve as a probe, stability of an emission current is higher than that of field emission electrons from the {310} plane, and the stability can be further improved by increasing the ratio J/It of the radiation angle current density J(μA/sr) to the total current It (μA). As a result of further studies conducted by the inventors, it has been found that, similar to the hexaboride single crystal such as CeB, in a transition metal carbide single crystal such as HfC, ZrC, or TiC which is a compound having a work function lower than W and having the same cubic crystal structure, when field emission electrons from the (100) plane served as a probe, stability of emission current was also higher than that of the field emission electrons from the {310} plane. In the field emission electron source using such low work function materials, when the ratio J/It of the radiation angle current density J(μA/sr) to the total current It (μA) is increased, generation of an electron beam stimulation desorption gas can be prevented and stability is improved. Therefore, it is expected to further increase J/It and it is expected to develop a new technique to prevent gas adsorption to an electron emission surface.

An object of the invention is to solve the above problems and provide an electron beam device such as an electron microscope that uses a stable electron beam emitted from a local region of an electron emission surface having a desired shape that is less likely to be affected by gas adsorption and whose changes over time are stabilized, that further uses a method of preventing mixing of an unstable electron beam emitted from a region other than the emission surface, that provides a field emission electron source of a hexaboride single crystal or a transition metal carbide single crystal having both monochromaticity and long-term stability of an emission current, and that can be used for various applications requiring high resolution and long-term stability.

In order to solve the above problems, a field emission electron source according to the invention is characterized in that a first (100) plane top facet is formed at a distal end of a tip of a hexaboride single crystal or a transition metal carbide single crystal having a <100> axis for forming the field emission electron source, the first (100) plane top facet is surrounded by side facets that include at least four {n11} planes and at least four {n10} planes, n being an integer of 1, 2, or 3, and of which a total area of {n11} plane side facets is larger than a total area of {n10} side facets, further a microcrystal having a second (100) plane top facet is formed on a plane of the first (100) plane top facet, and electrons are emitted from the second (100) plane top facet.

Here, the hexaboride single crystal is a single crystal having LaBor CeBas a main component, and the transition metal carbide single crystal is a single crystal having HfC, ZrC or TiC as a main component, and these single crystals are suitable materials for the invention since these single crystals have a low work function, high heat resistance and a cubic crystal structure necessary for forming the microcrystal having the second (100) plane top facet at the distal end of the tip.

The above problems can also be effectively solved by setting the second (100) plane top facet to be smaller than the first (100) plane top facet, setting a ratio of one side of the second (100) plane top facet to one side of the first (100) plane top facet in a range of 0.05 to 0.35, forming the microcrystal that forms the second (100) plane top facet into a cubic shape in which side planes are formed by a {100} plane group, or a trapezoidal shape in which side planes are formed by a {111} plane group, setting one side of the second (100) plane top facet in a range of 10 nm to 60 nm, and setting a height of the microcrystal to be 0.7 times or more a length of one side of the second (100) plane top facet.

The above problems can be effectively solved by setting one side of the first (100) plane top facet to be 1.5 μm or less and having four or less stages, so that it is possible to prevent surface diffusion of an adsorption gas from side walls of the tip toward an electron emission surface.

In order to solve the above problems, a method of producing an electron source according to the invention includes processing a distal end portion of a rod of a hexaboride single crystal or a transition metal carbide single crystal having a <100> orientation into a tip with the distal end portion of the rod being shaped into a cone shape by electrolytic polishing, and applying a high electric field with the tip having a positive polarity while heating the tip to form, at a distal end of the tip, a first (100) plane top facet surrounded by side facets that include at least four {n11} planes and at least four {n10} planes, n being an integer of 1, 2, or 3, and of which a total area of {n11} plane side facets is larger than a total area of {n10} side facets, and thereafter mainly lowering the electric field to form a microcrystal having a second (100) plane top facet on a plane of the first (100) plane top facet at the distal end of the tip.

Further, in order to solve the above problems, an electron beam device according to the invention includes an electron source, a sample stage on which a sample is placed, and an electron optical system configured to focus electrons emitted from the electron source into a beam shape and irradiate the sample on the sample stage with the focused electrons, in which a field emission electron source includes a distal end of a tip of a hexaboride single crystal or a transition metal carbide single crystal having a <100> axis, a first (100) plane top facet is formed at a distal end of the tip, the first (100) plane top facet is surrounded by side facets that include at least four {n11} planes and at least four {n10} planes, n being an integer of 1, 2, or 3, and of which a total area of {n11} plane side facets is larger than a total area of {n10} plane side facets, and further a microcrystal having a second (100) plane top facet is formed on a plane of the first (100) plane top facet.

According to the invention, it is possible to provide a new field emission electron source having monochromaticity and long-term stability of an emission current, and an electron beam device such as an electron microscope that includes the field emission electron source and can be used for various applications requiring high resolution and long-term stability.

In the following description, a crystal plane and a crystal orientation are represented according to Miller indices, a single plane is indicated by ( ) and an equivalent plane group is indicated by { }. A crystal axis direction is indicated by [ ] and an axial direction equivalent thereto is indicated by < >.

As a result of intensive studies of the inventors, according to an aspect of the invention, a field emission electron source is used in which a first (100) plane top facet is formed at a distal end of a tip of a hexaboride single crystal or a transition metal carbide single crystal having a <100> axis for forming the field emission electron source, the first (100) plane top facet is surrounded by side facets that include at least four {n11} planes and at least four {n10} planes, n being an integer of 1, 2, or 3, and of which a total area of {n11} plane side facets is larger than a total area of {n10} side facets, further a microcrystal having a second (100) plane top facet is formed on a plane of the first (100) plane top facet, and electrons are emitted from the second (100) plane top facet. A reason will be described below.

As disclosed by inventors in PTL 3, a field emission electron source using a (100) plane has higher stability of an emission current than a field emission electron source using a (310) plane, but there is a problem that a ratio of a radiation angle current density J(μA/sr) to a total current It (μA) is low. As a solution to this problem, it is required to reduce an amount of electrons emitted from a periphery other than the (100) plane top facet. Therefore, by developing a new producing method, a tip of an electron source was successfully produced in which a (100) plane top facet was formed at a distal end of a tip of a single crystal with a <100> axis, the (100) plane top facet was surrounded by side facets that include at least four {n11} planes and at least four {n10} planes, n being an integer of 1, 2, or 3, and of which a total area of the {n11} side facets having a high work function is larger than a total area of the {n10} side facets having a low work function, and the ratio J/It of the radiation angle current density J(μA/sr) to the total current It (μA) was increased to 2.6 to 4.

However, in this structure, a (100) plane top facet was flat, and although an area of the (100) plane top facet was reduced, {n10} planes such as {310} planes of side facets adjacent to the (100) plane top facet were sharpened, and thus an electric field was likely to be concentrated, and it was difficult to further increase J/It.

Therefore, by further improving a producing method disclosed in PTL 3, in an embodiment of the invention, it was found that, by forming a first (100) plane top facet surrounded by side facets that include at least four {n11} planes and at least four {n10} planes, n being an integer of 1, 2, or 3, and of which a total area of the {n11} side facets is larger than a total area of the {n10} side facets, and further forming a microcrystal having a second (100) plane top facet on a plane of the first (100) plane top facet, an electric field can be concentrated on the second (100) plane top facet, an amount of electrons emitted from the (100) plane having high stability can be increased, and an electric field applied to the {n10} planes of the side facets can be reduced relatively, so that unnecessary current emission outside an optical axis can be prevented and J/It can be further increased.

Hereinafter, embodiments will be described with reference to the drawings. Although a scanning electron microscope (SEM) is described as an example of an electron beam device according to the embodiments, the invention is not limited thereto. The invention can be applied to a transmission electron microscope (TEM), a scanning transmission electron microscope (STEM), an electron beam exposure device, an electron beam device including an electron beam type 3D printer, an X-ray tube, and the like. In the following drawings, a scale of each configuration is appropriately changed in order to facilitate understanding of a configuration of the invention.

In Embodiment 1, a structure of a field emission electron source according to the invention (hereinafter, may be simply referred to as an electron source) and an assembling method of the field emission electron source will be described with reference to. A method for forming first and second (100) plane top facets at a distal end of a tip, which is the feature of the invention, will be described in Embodiment 2 and subsequent embodiments.

First, a hexaboride single crystal or a transition metal carbide single crystal of rare earth is used as a material of the electron source according to the invention. Specifically, lanthanide elements such as La, Ce, Pr, Nd, Sm, Eu, and Gd can be used as the hexaboride single crystal, and are represented by chemical formulas LaB, CeB, PrB, NdB, SmB, EuB, and GdB, respectively.is a schematic view showing a unit lattice. The unit latticehas a cubic crystal structure in which a block of six boron atomsis positioned at a body center of a simple cubic lattice of metal atoms. These materials generally have a high melting point (for example, LaB: 2483 K, CeB: 2463 K), low vapor pressure, high hardness, high resistance to ion bombardment, and a lower work function than W (for example, LaB, CeB: about 2.6 eV<W: about 4.3 eV), making these materials suitable as materials of an electron source. Among them, LaBand CeBare materials that are widely used as a material of a thermal electron source. In particular, a single crystal having LaBor CeBas a main component can be effectively used as the hexaboride single crystal.

On the other hand, a single crystal having HfC, ZrC, TiC as a main component can be effectively used as the transition metal carbide single crystal.is a schematic view showing a unit lattice. The unit latticehas a cubic crystal structure in which carbon atomsand the metal atomsare alternately arranged. As compared with the hexaboride single crystal, these materials of the transition metal carbide single crystal have a higher melting point (for example, HfC: 4163 K, ZrC: 3805 K, TiC: 3443 K), lower vapor pressure, higher hardness, higher resistance to ion bombardment, and a lower work function than W (for example, HfC, ZrC, TiC: about 3.3 eV), making these materials suitable as materials of an electron source.

In the present embodiment, an example in which CeBof the hexaboride single crystal and HfC of the transition metal carbide single crystal are used will be mainly described. Among materials of the hexaboride single crystal of rare earth, CeBhas f-electrons with strong energy localization and a high state density just below the Fermi level, and has a high electron density for supplying an emission current, so that CeBis particularly suitable as a material of the hexaboride single crystal used to produce a field emission electron source. On the other hand, HfC is a material having a highest melting point among materials of the transition metal carbide single crystal, and HfC is particularly suitable as a material for producing a field emission electron source.

Although a producing method according to the invention is generally the same for the hexaboride single crystal and the transition metal carbide single crystal, due to differences in a melting point and reactivity with other materials constituting an electron source (especially a metal tube that holds a single crystal), there are some slight differences in a processing temperature and an assembly process of the electron source, and the differences will be described as appropriate.

As shown in, the hexaboride single crystal or the transition metal carbide single crystal can be grown by melt (liquid phase) crystal growth using, for example, a floating zone method, to produce a large single crystalwith a diameter of several mm and a length of several mm to several tens of mm at which the crystal grows in a crystal axis direction perpendicular to a (100) plane of a crystal plane on which the crystal preferentially grows. The single crystalis used by cutting out, by cutting or polishing, a rodthat has a length of several mm and is a square pillar having one side of 100 μm or a cylindrical pillar having a diameter of 100 μm. In the present embodiment, the rodthat is a square pillar having one side of 200 μm and a length of 5 mm or a cylindrical pillar having a diameter of 280 μm and a length of 5 mm was used. A longitudinal direction of the rodis a orientation.

A crystal structure of the above-described hexaboride single crystal or transition metal carbide single crystal is a cubic simple cubic lattice as shown in, and a (100) plane, a (010) plane, a (001) plane, a crystal axis, a crystal axis, a crystal axis, and the like are equivalent, and the effect is the same regardless of which plane or axis is used. Therefore, in the following description, an equivalent plane group is described as {100} or the like, and an equivalent axis group is described as <100> or the like.

Next, a bonding method for holding the rodof the hexaboride single crystal or the transition metal carbide single crystal and attaching a filament for heating will be described with reference to. In the electron source according to the present embodiment, the rodof the hexaboride single crystal or the transition metal carbide is provided inside a metal tubemade of tantalum, niobium, or the like.

A material of the metal tubeused to join the rodof the hexaboride single crystal or the transition metal carbide single crystal is preferably a high melting point metal such as tantalum or niobium, which is highly ductile, can be easily stretched to form the minute metal tube, and on which a recessed portion can be easily processed as will be described below. In the present embodiment, the minute metal tubehaving an outer diameter Φ of 500 μm, an inner diameter Φ of 320 μm, a thickness of 90 μm, and a length of 5 mm was produced using, for example, tantalum.

Next, a method of bonding the rodof the transition metal carbide single crystal or the hexaboride single crystal using the metal tubewill be described. First, as shown in, a guide pinis placed vertically on a base, and the guide pinhas a diameter of 300 μm that enables the guide pinto enter an inner diameter of the metal tubeand has a length of 1 mm to 3 mm. The guide pinis inserted into the metal tube, and the metal tubeis placed p vertically on the base. Subsequently, when the hexaboride single crystal is used, a pasteis filled in the metal tubefrom above, and the pasteis obtained by mixing nanoparticles such as boron tetracarbide BC having an average particle diameter of 0.01 μm to 0.1 μm with a carbon resin such as a furan resin. Here, nanoparticles having an average particle diameter of 0.05 μm were used. On the other hand, when the transition metal carbide single crystal is used, since the transition metal carbide single crystal does not react with the metal tubemade of tantalum, niobium, or the like at a high temperature, it is not necessary to perform the filling step.

Further, the rodof the hexaboride single crystal or the transition metal carbide single crystal is inserted into the metal tubefrom above. A protruding length h at which the rodof the hexaboride single crystal or the transition metal carbide single crystal protrudes from an inner side of the metal tubecan be controlled by the guide pin. In the present embodiment, in order to cut a distal end on one side of the rodof the hexaboride single crystal or the transition metal carbide single crystal by electrolytic polishing, which will be described later with reference to, the protruding length h is set to be 2 mm to 3 mm.

Subsequently, as shown in, the rodof the hexaboride single crystal or the transition metal carbide single crystal and the metal tubeare pressure-welded to each other with a special tool developed by the inventors from two axes and four directions perpendicular to a vertical direction of the rod. In order to simplify description,shows only bladesof a pressure welding tool. A pair of upper and lower protrusionsfor forming recessed portions in the metal tubeare provided at a distal end of the bladeof the pressure welding tool. The bladeof the pressure welding tool is brought close to the metal tubeat equal strokes from the two axes and four directions, and the metal tubeis crushed by the protrusionsfrom an outer periphery, thereby forming a plurality of recessed portionsin the metal tubeas shown in.

During an operation, a positional relationship between the metal tubeand the rodof the hexaboride single crystal or the transition metal carbide single crystal is confirmed using a stereo microscope, and a rotation axis of the rodof the hexaboride single crystal or the transition metal carbide single crystal is appropriately adjusted so that each side surface of the rodof the square pillar hexaboride single crystal or transition metal carbide single crystal coincides with a stroke direction of the bladeof the tool. Accordingly, the plurality of recessed portionsare formed in a manner of surrounding a center axis from the outer periphery of the metal tube, and a bottom portion of each of the recessed portionsis pushed against and brought into contact with an outer peripheral surface of the rodof the hexaboride single crystal or the transition metal carbide single crystal, so that the rodof the hexaboride single crystal or the transition metal carbide single crystal can be fixed by being automatically aligned with the center axis of the metal tube.

are schematic views showing the rodof the hexaboride single crystal or the transition metal carbide single crystal and the metal tubethat are joined by the method according to the present embodiment.is a plan view showing a joint portion as viewed from a distal end side of the rod,is a perspective view showing the rod, andis a cross-sectional view taken along a vertical direction of the rod.

When the bonding method is used, the metal tubeand the rodof the hexaboride single crystal or the transition metal carbide single crystal can be evenly pressure-welded from the two axes and four directions, and strong bonding can be obtained mechanically. In addition, since the bladesare brought close to the metal tubeat equal strokes from the two axes and four directions and the metal tubeis crushed from the outer periphery, the rodof the square pillar hexaboride single crystal or transition metal carbide single crystal can be joined to the metal tubeby being automatically aligned with the center axis of the metal tube. Since assembly precision is improved, it is easier to align an axis of the electron source, yield is also improved. Further, since the rodcan be joined at upper and lower two positions in an axial direction, the rodcan be prevented from being inclined at a joint portion, and accuracy of an axial alignment can be further increased.

Further, when the hexaboride single crystal is used, the pasteobtained by mixing nanoparticles of boron tetracarbide BAC with a carbon resin such as a furan resin is flexibly deformed at the time of pressure welding, and a space between the deformed metal tubeand the rodof the hexaboride single crystal is filled without a gap. Since small nanoparticles having an average particle diameter of 0.1 μm or less are used as the paste, the rodof the hexaboride single crystal is not damaged or broken at the time of pressure welding, and yield in a pressure welding step can be improved. The reason why the average particle diameter of the nanoparticles is set to be 0.01 μm or more is that when the average particle diameter is too small, an apparent volume of BC powder increases, mixing for the paste becomes difficult, production of the nanoparticles becomes difficult, and cost increases.

After the metal tubeis pressure-weld to the rodof the hexaboride single crystal or the transition metal carbide single crystal, a portion-indicated by a dotted line of the metal tubeinto which the guide pinis inserted is not necessary, and thus, after the metal tubeis removed from the guide pin, the portion-is cut out with a cutter to reduce heat capacity of the metal tube. Thereafter, in the case of the hexaboride single crystal, the pasteis heated in the air to become harden, and then heated at a high temperature of 1000° C. or higher in a vacuum for several hours to carbonize the paste. Accordingly, degassing from the pasteis eliminated, and a reaction barrier layer for preventing reaction at high temperature between the metal tubemade of tantalum or the like and the rodof the hexaboride single crystal can be formed.

Subsequently, as shown in, a filamentmade of tungsten or the like is directly spot-welded to the metal tubeto which the rodof the hexaboride single crystal or the transition metal carbide single crystal is joined. Further, both ends of the filamentare spot-welded to a pair of electrodesfixed to a stemto form a structurewhich is a prototype of an electron source. Since the structureis formed by bonding metals, strong bonding can be obtained by spot welding.

A specific example of welding processing for forming the structurewill be described with reference to. When the filamentmade of tungsten or the like is directly spot-welded to the metal tubeto which the rodof the hexaboride single crystal or the transition metal carbide single crystal is joined, a positioning jigas shown inis used. First, the filamentmade of tungsten or the like is accurately aligned with the metal tubeusing a positioning jig-, and the metal tubeand the filamentare spot-welded.

Subsequently, as shown in, the metal tubeto which the filamentis spot-welded and the stemare accurately aligned by using a positioning jig-, and the filamentand the pair of electrodesfixed to the stemare spot-welded to form the structure. As described above, by using the positioning jigs-and-, since a center axis of the metal tube, a center axis of the rodof the hexaboride single crystal or the transition metal carbide single crystal are aligned with the center of the pair of electrodesfixed to the stemin a stage when the rodand the stemare assembled as the structure, the structureallows for highly accurate centering.

In the embodiment described above, the rodof the hexaboride single crystal or the transition metal carbide single crystal that is cut into a square pillar shape is used as a structural component of the structure. The rodof the hexaboride single crystal or the transition metal carbide single crystal may be processed into a cylindrical pillar as shown in.show an example when a rod-of a cylindrical pillar hexaboride single crystal or transition metal carbide single crystal is used. In a case where the rod-of the cylindrical pillar hexaboride single crystal or transition metal carbide single crystal is joined to the metal tube, it is sufficient that the rod-and the metal tubeare pressure-welded using a special tool developed in the present embodiment from three axes and three directions at equal intervals in a plane perpendicular to the vertical direction of the rod-of the hexaboride single crystal or the transition metal carbide single crystal. In particular, in the case of the rodof the transition metal carbide single crystal, since the rodcan be pressure-welded without using the paste, it is preferable to process the rodinto a cylindrical pillar having a similar shape to the inner diameter of the metal tube, which makes pressure welding easier.

is a plan view showing a joint portion as viewed from a distal end side of the rod-,is a perspective view showing the rod-, andis a cross-sectional view taken along a vertical direction of the rod-.show a state after cutting a portion corresponding to the portion-that is unnecessary after the rodof the hexaboride single crystal or the transition metal carbide single crystal is pressure-welded to the metal tubeas described with reference to.

It is needless to say that the metal tubeand the rod-of the cylindrical pillar hexaboride single crystal or transition metal carbide single crystal may be joined by being pressure-welded to each other from two axes and four directions in a similar manner to the case of the rodof the square pillar hexaboride single crystal or transition metal carbide single crystal as described with reference toand.

Subsequently, in the structure, a distal end of a portion of the rodof the hexaboride single crystal or the transition metal carbide single crystal where the rodprotrudes out from the metal tubeis reduced in diameter in a cone shape by electrolytic polishing. The electrolytic polishing is performed by dipping a distal end portion of the rodof the hexaboride single crystal or the transition metal carbide single crystal that is assembled as shown inin an electrolytic solutionsuch as nitric acid in a container, and applying a voltage from a power supplyof an alternating current or a direct current to a space between the power supplyand a ring-shaped counter electrodemade of platinum or the like.